Conductive metal ball bonding is one of the methods used in the electronics industry for electrical connection of conductive leads to electronic devices such as devices fabricated from a wafer (e.g. chips). Typically, the conductive leads and corresponding bond pads on the electronic devices are coated with the same conductive metal as the conductive metal balls used for electrical connection, to facilitate joining of the metal through the addition of energy. Typically, the conductive metal coatings and the conductive metal balls comprise gold, and the added energy comprises ultrasonic waves and/or heating. For example, U.S. Pat. No. 4,925,083 describes a method and apparatus for gold ball bonding.
One of the applications for gold ball bonding (GBB) is the fabrication of head-gimbal assemblies (HGAs) for hard disk drives. HGAs include heads for reading and writing data from and to a disk. In magnetic recording applications, the head typically includes a slider and a magnetic transducer that comprises a writer and a read element. The slider is cut from a ceramic wafer (typically AlTiC) upon which the magnetic transducer is fabricated by photolithographic and vacuum deposition and etching techniques. In optical and magneto-optical recording applications, the head may include a mirror and an objective lens for focusing laser light on to an adjacent disk surface. During operation, an “air bearing surface” (ABS) of the slider is separated from the disk by a thin gas lubrication film that is typically referred to as an “air bearing.”
For example, FIG. 1 depicts a distal portion of a contemporary HGA 100 that includes magnetic recording head 102. Head 102 comprises air bearing slider 104 and magnetic transducer 106. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer is typically magnetoresistive (MR). The head 102 is adhered to a gimbal 112 of a suspension assembly 110. Suspension assembly 110 also includes a load beam 114, a bend region (not shown), and a swage plate (not shown). The suspension assembly 110 acts to preload the air bearing slider against the surface of the disk.
The suspension assembly of an HGA constrains the motion of the head along or about certain axes, while being compliant to motion of the head along or about other axes. For example, the suspension assembly includes a load beam that extends from a bend region that permits motion towards and away from the disk, while preloading the head against the disk and constraining translation parallel to the surface of the disk. Also for example, the suspension assembly includes a dimple about which the head may pivot, but which constrains translation between the head and the load beam. Also for example, the suspension assembly includes a gimbal to which the head is bonded, the gimbal being compliant to pitch and roll motions of the head but constraining yaw motions of the head.
In modern disk drives, the gimbal also performs the function of carrying leads for electrical connection to the head. Typically, the modern gimbal comprises a laminated structure having a structural layer and conductive leads, separated by a dielectric layer. The leads terminate adjacent a face of the slider that includes bond pads for the magnetic transducer device. The face of the slider containing the bond pads is typically orthogonal to the plane of the gimbal's conductive leads. Therefore corresponding electrical connections must be made through a 90° angle, which is a requirement that can be satisfied by conductive metal ball bonding.
For example, in FIG. 1 gimbal traces 116 are electrically connected to bond pads 108 on the head by conductive balls 120. Note that the HGA of FIG. 1 is shown in a state of incomplete fabrication because not all of bond pads 108 have yet been connected to gimbal traces 116 by conductive balls 120.
The bond pads on the slider, and the regions of termination of the gimbal's conductive leads, are often coated with gold (e.g. by plating), with gold balls used to connect the gimbal's conductive leads to the bond pads of the magnetic transducer device. The gold balls can be fused to the gold coatings on the gimbal's conductive leads and the bond pads of the magnetic transducer device using ultrasonic energy.
However, modern magnetic transducer devices used in read heads for disk drives are extremely sensitive to electrostatic discharge (ESD). For example, MR read elements of all types are easily damaged by ESD, and tunneling MR read elements (a.k.a. “TMR” read elements) in particular are very easily damaged by ESD even at modest voltages. Prior art methods for protecting MR read elements from damage due to ESD have had limited success for various reasons. For example, some prior art methods have undesirably and/or excessively complicated wafer-processing steps during manufacture of the magnetic recording heads. Other prior art methods are too inconvenient to be used in a high-volume manufacturing environment, or require equipment that is too costly to be implemented in a high-volume manufacturing environment. Other prior art methods offer insufficient ESD protection and/or do not afford protection early enough in the manufacturing process. For example, TMR read elements are particularly vulnerable to ESD damage during the manufacturing process before the bond pads of the head are electrically connected to the gimbal's conductive leads (which may themselves connect to some protective circuitry).
Consequently, ESD damage to magnetic transducer devices during the GBB process of high-volume HGA manufacture can significantly reduce manufacturing yield. Moreover, if such damage is not detected right away, the damaged devices may be assembled into more costly components later in the manufacturing process, such as finished head stack assemblies (HSAs), each containing many HGAs, or even head disk assemblies (HDAs), each including an HSA as well as other costly components (e.g. one or more disks, a spindle motor, etc). Of course, if the damaged magnetic transducer device is assembled into one of these more costly components later in the manufacturing process, it is likely to render the more costly component inoperable, leading to even greater waste, and lowering manufacturing yield still further. Thus, there is a need in the art for a practical method to detect ESD during metal ball bonding in a high-volume manufacturing environment.